Heligophagus waandersi
In the Mekong mainstream, Heligophagus waandersi is distributed from the Mekong delta to Bokeo in northern Lao PDR. Apart from the Mekong River, the species also occurs in the Chao Phraya basin in Thailand and in rivers on the island of Sumatra, Indonesia (Roberts and Vidthayanon, 1991; Rainboth, 1996).
The Khone Falls appear, to a great extent, to act as a barrier for this species, i.e., migration patterns above and below the Khone Falls are significantly different.
Below the Khone Falls, Heligophagus waandersi migrates upstream from October to February. From May to July, the species migrates downstream, at least from Sambor to Muk Kompul in Kandal province of Cambodia. Interestingly, there were no reports on migratory behaviour from Sambor to the Khone Falls, although the species was reported to be common along that stretch. Similarly, there were no reports from the Mekong delta on migratory behaviour, although five stations reported that the species occurs in the delta area.
Above the Khone Falls, there appear to be two upstream movements, one during the late dry season from March to May (mainly along the stretch from Paksan to Loei) and one during the early rainy season, from May to August.
Fifteen reports were received on eggs having been observed in the abdomen of fish: nine reports in June, seven in May and five each in March, April and July. The reports provided an indication that the main spawning season is from May to June. There were no indications of differences in the timing of spawning, except for one report from Dong Thap province in Viet Nam noting that the species spawned locally throughout the year as "fry are observed all year round".
South of the Khone Falls, five stations reported juveniles of 2-4 cm size from Kompong Cham province, Cambodia, to Can Tho and Dong Thap provinces in Viet Nam.
Above the Khone Falls, 2-4 cm juveniles were reported in Nong Khai, Mukdahan and Ubon Ratchatani provinces of Thailand. There were no reports from either the Thai or Lao side on juveniles upstream of Nong Khai province
Hypothesis:
There are several populations of Heligophagus waandersi in
the Mekong River, especially above the Khone Falls.
The Khone Falls forms a barrier to the movement of this species and the migration patterns below and above that point are significantly different.
Above the Khone Falls, several populations exist, each undertaking a relatively short upstream migration during the late dry season and/or early flood season. Those movements appear to be spawning migrations, although they may also involve trophic migrations of sub-adults. Some of the migrating populations migrate into major tributaries (e.g., the Nam Ngum and Songkhram rivers, as confirmed during the trial survey of the tributaries).
Below the Khone Falls, Heligophagus waandersi migrates upstream from the onset of the dry season until the late dry season, and downstream during the onset of the flood season. There may also be several sub-populations along that stretch, e.g., a relatively sedentary population may exist in the Mekong delta